Well, the Salt Creature was the last of it's kind. It died at the end of The Man Trap.
The Horta only lived on Janus 6 I believe. So they'd have to revisit that planet in the spin-off's.
Trelane was punished and probably couldn't make any more planets!

The idea that Trelane is Q has never been proved 100%. As mentioned earlier, William Campbell himself felt Trelane was not the same character as Q.
The Mugatu never appeared in any of the spin-offs either.
The only claasic creature to appear on a spin-off is the Gorn. He was in Enterprise, In a Mirror Darkly. And I'm not considering the Andorians or Tellarites as creatures.
Here's a question:
Name a character in either the 22nd, 23rd or 24th Century that has driven a 20th Century automobile. See if you can name the three I have in mind.